One group of theft-preventing systems uses antitheft tags attached to articles. Before leaving the shop the antitheft tags must be removed or deactivated by the staff in the shop. At the exit doorway a detector is arranged for detecting the presence of antitheft tags in a detection zone near the doorway. If an article is brought out of the shop with an antitheft tag still attached, the detector will detect the antitheft tag and give an alarm or other indication, and appropriate action can be taken.
Several theft-preventing systems exist. In one system the antitheft tags have an electrical resonance circuit with an inductor coil and a capacitor tuned to a predetermined resonance frequency, typically in the MHz range. At the exit doorway a transmitter antenna emits an electromagnetic signal comprising the resonance frequency of the antitheft tags into the detection zone. The signal from the transmitter antenna will excite possible antitheft tags in the detection zone to “ring” at their resonance frequencies. A receiver antenna will detect such ringing. Antitheft tags of this system can have wide tolerances to the accuracy of the resonance frequency, and the electromagnetic transmitter antenna can emit a broadband signal covering the interval of tolerance on the resonance frequency of the tags.
Another known system uses antitheft tags with an RFID chip containing data that can be detected by the system that emits an interrogation signal. RFID tags in the detection zone respond to the interrogation signal by emitting a signal with some or all data, and possibly revealing their identity.
A further known system uses a diode or other electrically non-linear element in the tags. Such tags respond to the electromagnetic signal from the transmitter antenna by emitting harmonic frequencies that are detected and taken as an indication of an antitheft tag being in the detection zone.
A known way of circumventing these tag-based antitheft systems is to coat a bag or other type of container with aluminium foil or another metal such as for example tin-foil and put the tagged item into the bag. Thereby the tagged item will be in a Faraday cage and as the detection zone is passed the signal from the tagged item will not reach the detection system and thus the tagged item will be removed from the shop without any alarm being given off. The abovementioned metal-coated bag is known as a booster bag. Alternatively or additionally, a shoplifter may coat for example a piece of clothing such as for example an overcoat and use the piece of clothing to shield the signal from the tagged item from reaching the detection system. Such a metal coated piece of clothing may be comprised by the term booster bag in this application.
A solution to this problem is to have a metal detector system in the detection zone or in proximity to the detection zone, either as a stand alone system or integrated into the tag-based detection system. The metal detector is sensitive to movements of metal objects in the detection zone of the metal detector. Thereby, metal objects passing the detection zone can be detected. The metal detection systems will typically be based on a magnetic field transmitter and a magnetic field detector/receiver.
The abovementioned magnetic field based metal detector systems can present problems when used near a door with two or more metal parts that are joined, such as a door with a frame of aluminium profiles mechanically joined at the corners. The magnetic field from the transmitter will couple through the metal frame of the door and yield a contribution to the magnetic field detected at the receiver. As the door opens to a customer entering or leaving the store, the coupling of the magnetic field though the doorframe will be broken and the signal supplied by the doorframe at the receiver will disappear. The metal detection system will detect this, due to a decrease in the detected magnetic field strength at the receiver, as a moving metal object entering the detection zone and may thus cause a false alarm. As the door closes after a customer entering or leaving the store, the magnetic field may again couple through the door and the signal supplied by the doorframe at the receiver will reappear. The metal detection system may also detect this, due to an increase in the detected magnetic field strength at the receiver, as a moving metal object out of the detection zone and thus cause a false alarm.
Further, the door comprising one or more metal parts opening and closing to customers may—also—present a problem to the tag-based antitheft system. Such a door may act as an active tag and therefore an opening and/or closing door comprising one or more metal parts can also influence the tag-based detector system.
Thus, in an antitheft detection system comprising a tag-based antitheft system and a metal detector, the problem of a door with metal parts is twofold: Firstly, the metal detector may falsely detect the movement of the door as a metal object in the detection zone of the metal detector yielding a false alarm. Secondly, the opening and/or closing door with metal parts may act as an active tag influencing the tag-based detection system.